shadow

Area of darkness behind an opaque object that cannot be reached by some or all of the light coming from a light source in front. Its presence may be explained in terms of light rays travelling in straight lines and being unable to bend around obstacles. The light in front of the object is blocked. A point source of light produces an umbra, a completely black shadow with sharp edges. An extended source of light produces both a central umbra and a penumbra, a region of semidarkness with blurred edges where darkness gives way to light.

Eclipses are caused by the Earth passing into the Moon's shadow or the Moon passing into the Earth's shadow. During a solar eclipse the Moon forms a shadow on the Earth as it lines up between the Sun and the Earth. A total eclipse occurs when the Moon completely blocks out the light from the Sun. This is called the umbra. A partial eclipse occurs when some of the light from the Sun reaches the Earth around the edges of the Moon. This is called a penumbra.